One wonders if the Google taxis will come with a Google Translate applet that will talk to you in a different language just to keep the flavor of original taxicabs.
I'm cautiously optimistic about this concept but I think that, similar to the Amazon "Drone-A-Gram" thing, it only really lends itself to high-density areas. For a place like Montana, people are going to keep their trucks because a "drive into town" could be 50 miles or more.
The other issues are price-point and accessibility. Someone made the analogy to cell phones and how widespread adoption was 15-20 years behind introduction.
Things really start to accelerate around 1994 and begin flattening out in 2006 (probably more due to market saturation than pricepoint).
In 1994, the biggest seller was the Motorola StarTAC which was $1,000 a pop but was also a mere 3.1 ounces, unlike its predecessors which had been in the range of 1-2 POUNDS. So there's the accessibility part.
By 2001-2002, when you hit that 50/50 split between landlines and mobiles, you could snag a Palm Treo or a Nokia for about $180. That's where the pricepoint is hit, I think. Of course, calculating a true pricepoint is tough because of the varying prices for plans (where most of the cost has been offloaded over time). People drop their landlines for mobiles when the cost of the phone (prorated over expected life of the phone) + monthly plan + perceived utility cost >= monthly cost of landline. In other words, you might wind up paying $10 more a month in actual cost, but the utility cost of being able to have your phone with you anywhere is greater than $10/month to you, so you switch.
I think the accessibility part is going to be pretty damn high initially for these. There's a large base of Google users and a large base of mobile phone users, and they overlap pretty well. So the early adopter portion of the curve might be pretty sharp. It's getting the price point down for everyone else that will take some time. People in San Fran, NYC, Los Angeles.....they might be willing to pay $2.50 a mile or $20 a ride or what have you -- still beats the price of car ownership in big cities like that (especially New York, where travel distances are relatively short and garage/parking space is super-expensive).